Bug 2223046

This bug was initially created as a copy of Bug #2180955

I am copying this bug because: 

The original issue is cited in an errata for fast datapath RHEL 8. This issue will be cited in an errata for fast datapath RHEL 9.

Description of problem:

When a port is multichassis (requested-chassis is a comma separated list), if the LS that the port belongs to has a localnet port, traffic to and from the multichassis port is tunneled anyway. (This is done to guarantee delivery of packets destined to the port MAC address to all its locations.)

This enforced tunneling may be a problem if the effective MTU for the ports becomes different from the theoretical MTU of the physical network that underlies the LS (defined by MTU of localnet port in the same switch). In this case, the port should not communicate with the outside world using the max MTU.

The proposal here is for OVN controller to set up ICMP Path Discovery replies to oversized packets received from a multichassis port, so that the port owner is aware of the change in circumstances and can adequately adjust their effective MTU.

The problem was originally discussed in ovs-dev ML: https://www.mail-archive.com/ovs-dev@openvswitch.org/msg68204.html

Implementation was proposed at: https://mail.openvswitch.org/pipermail/ovs-dev/2022-November/398981.html

This bug is to take the patch over and get it tested / merged in OVN.

The bug affects OSP live migration scenario for VMs attached to physical networks (=switches with localnet port).

I expect this bug to become a blocker for OSP 17.1 because of its effect on live migration scenario.